Merge pull request #1 from Jasig/master

pull upstream
Merge remote-tracking branch 'upstream/master'
makes the plugin Compass aware

Succesfully builds the sample project mprins/maven-compass-example.

Note that for the build we're now relying on the Compass gem and the transitive dependency that has on the Sass gem.

For the Mojo, since buildDirectory has a default value there is no sense in that being a required parameter.